#include<iostream>
#include<cmath>
#include <iomanip>
using namespace std;

int main(){
	double a,b,c;
	cin >> a >> b >> c;
	double p = (a + b + c) / 2;
	double s = sqrt(p * (p - a) * (p - b) * (p - c));
//	printf("%.1f",s);
	cout << fixed << setprecision(1) << s <<endl;
	return 0;
}
